XML Syntax (XML in a Nutshell, 2nd Edition)

For each section of this reference that maps directly to an XML language structure, an informal syntax reference describes theat structure's form. The following conventions are used with these syntax blocks:

Format	Meaning
DOCTYPE	Bold text indicates literal characters that must appear as written within the document (e.g., DOCTYPE).
encoding-name	Italicized text indicates that the user must replace the text with real data. The item indicates what type of data should be inserted (e.g., encoding-name = en-us).
\|	The vertical bar \| indicates that only one out of a list of possible values can be selected.
[ ]	Square brackets indicate that a particular portion of the syntax is optional.

20.3.1. Global Syntax Structures

Every XML document is broken into two primary sections: the prolog and the document element. A few documents may also have comments or processing instructions that follow the root element in a sort of epilog (an unofficial term). The prolog contains structural information about the particular type of XML document you are writing, including the XML declaration and document type declaration. The prolog is optional, and if a document does not need to be validated against a DTD, it can be omitted completely. The only required structure in a well-formed XML document is the top-level document element itself.

The following syntax structures are common to the entire XML document. Unless otherwise noted within a subsequent reference item, the following structures can appear anywhere within an XML document.

Whitespace

Whitespace is defined as a space, tab, or empty line (which is composed of a carriage return, line feed, or combination of the two). Whitespace serves the same purpose in XML as it does in most programming and natural languages: to separate tokens and language elements from one another. XML has simplified the task of determining which whitespace is significant to an application and which is not. To an XML parser, all whitespace in element content is significant and will be passed to the client application. Whitespace within tags--for instance, between attributes--is not significant. Consider the following example:

<p>  This sentence has extraneous 
  line breaks.</p>

After parsing, the character data from this example element is passed to the underlying application as:

   This sentence has extraneous
line breaks.

Though XML specifies that all whitespace in element content be preserved for use by the client application, an additional facility is available to the XML author to further hint that an element's character data's space and formatting should be preserved. For more information, see the discussion of the xml:space attribute in Special Attributes later in this chapter.

Names

To ease the burden of those who write XML parsers, XML names must adhere to the following lexical conventions:

Begin with a letter, _, or : character.
After the first character, be composed only of letters, digits, ., -, _, and : characters.

In this context, a letter is any Unicode character that matches the Letter production from the EBNF grammar at the end of this chapter.

According to the XML 1.0 specification, the : character may be used freely within names, although the character is now officially reserved as part of the Namespaces in XML recommendation. Even if a document does not use namespaces, the colon should still not be used within identifiers to maintain compatibility with namespace-aware parsers. See the Section 20.3.4 in this chapter for more information about how namespace-aware identifiers are formed.

Names should also avoid starting with the three-letter sequence X, M, L, unless specifically sanctioned by an XML specification.

Character References


&#decimal-number;
&#xhexadecimal-number;

All XML parsers are based on the Unicode character set, no matter what the external encoding of the XML file is. It is theoretically possible to author documents directly in Unicode, but many text-editing, storage, and delivery systems still use the ASCII character set. To allow XML authors to include Unicode characters in their documents' content without forcing them to abandon their existing editing tools, XML provides the character reference mechanism.

A character reference allows an author to insert a Unicode character by number into the output stream produced by the parser to an XML application. Consider an XML document that includes the following character data:

&#xa9; 2002 O'Reilly &#38; Associates

In this example, the parser would replace the character reference with the actual Unicode character and pass it to the client application:

© 2002 O'Reilly & Associates

Character references may not be used in element or attribute names, though they may be used in attribute values.

Predefined Entities

Besides user-defined entity references, XML includes the five named entity references shown in Table 20-1 that can be used without being declared. These references are a subset of those available in HTML documents.

Table 20-1. Predefined entities

Entity	Character	XML declaration
<	<	<!ENTITY lt "&#60;">
>	>	<!ENTITY gt ">">
&	&	<!ENTITY amp "&#38;">
'	"	<!ENTITY apos "'">
"	"	<!ENTITY quot """>

The < and & entities must be used wherever < or & appear in document content. The > entity is frequently used wherever > appears in document content, but is only mandatory to avoid putting the sequence ]]> into content. ' and " are generally used only within attribute values to avoid conflicts between the value and the quotes used to contain the value.

Though the parser must recognize these entities regardless of whether they have been declared, you can declare them in your DTD without generating errors.

The presence of these "special" predefined entities creates a conundrum within an XML document. Because it is possible to use these references without declaring them, it is possible to have a valid XML document that includes references to entities that were never declared. The XML specification actually encourages document authors to declare these entities to maintain the integrity of the entity declaration-reference rule. In practical terms, declaring these entities only adds unnecessary complexity to your document.

CDATA (Character Data) Sections

<![CDATA[unescaped character & markup data]]>

XML documents consist of markup and character data. The < or & characters cannot be included inside normal character data without using a character or entity reference, such as & or &. By using a reference, the resulting < and & characters are not recognized as markup by the parser, but will become part of the data stream to the parser's client application.

For large blocks of character data--particularly if the data contains markup, such as an HTML or XML fragment--the CDATA section can be used. Within a CDATA block, every character between the opening and closing tag is considered character data. Thus, special characters can be included in a CDATA section with impunity, except for the CDATA closing sequence, ]]>.

CDATA sections are very useful for tasks such as enclosing XML or HTML documents inside of tutorials explaining how to use markup, but it is difficult to process the contents of CDATA sections using XSLT, the DOM, or SAX as anything other than text.

NOTE: CDATA sections cannot be nested. The character sequence ]]> cannot appear within data that is being escaped, or the CDATA block will be closed prematurely. This situation should not be a problem ordinarily, but if an application includes XML documents as unparsed character data, it is important to be aware of this constraint. If it is necessary to include the CDATA closing sequence in the data, close the open CDATA section, include the closing characters using character references to escape them, then reopen the CDATA section to contain the rest of the character data.

Entity References

An XML entity can best be understood as a macro replacement facility, in which the replacement can be either parsed (the text becomes part of the XML document) or unparsed. If unparsed, the entity declaration points to external binary data that cannot be parsed. Additionally, the replacement text for parsed entities can come from a string or the contents of an external file. During parsing, a parsed entity reference is replaced by the substitution text that is specified in the entity declaration. The replacement text is then reparsed until no more entity or character references remain.

To simplify document parsing, two distinct types of entities are used in different situations: general and parameter. The basic syntax for referencing both entity types is almost identical, but specific rules apply to where each type can be used.

Parameter Entity References
%name;
When an XML parser encounters a parameter entity reference within a document's DTD, it replaces the reference with the entity's text. Whether the replacement text is included as a literal or included from an external entity, the parser continues parsing the replacement text as if it had always been a part of the document. This parsing has interesting implications for nested entity references:
<!ENTITY % YEAR "2001"> <!ENTITY COPYRIGHT "© %YEAR;"> . . . <copyright_notice>&COPYRIGHT;</copyright_notice>
After the necessary entity replacements are made, the previous example would yield the following canonical element:
<copyright_notice>© 2001</copyright_notice>
WARNING: XML treats parameter entity references differently depending on where they appear within the DTD. References within the literal value of an entity declaration (such as Copyright © %YEAR;) are valid only as part of the external subset. Within the internal subset, parameter entity references may occur only where a complete markup declaration could exist. In other words, within the internal subset, parameter references can be used only to include complete markup declarations.

Parameter entity references are recognized only within the DTD; therefore, the % character has no significance within character data and does not need to be escaped.

General Entity References
&name;
General entity references are recognized only within the parsed character data in the body of an XML document. They may appear within the parsed character data contained in an element start- and end-tag, or within the value of an attribute. They are not recognized within a document's DTD (except inside default values for attributes) or within CDATA sections.
NOTE: The sequence of operations that occurs when a parsed general entity is included by the XML parser can lead to interesting side effects. An entity's replacement text is, in turn, read by the parser. If character or general entity replacements exist in the entity replacement text, they are also parsed and included as parsing continues.

Comments

<!-- comment text -->

Comments can appear anywhere in your document or DTD, outside of other markup tags. XML parsers are not required to preserve contents of comment blocks, so they should be used only to store information that is not a part of your application. In reality, most information you might consider storing in a comment block probably should be made an official part of your XML application. Rather than storing data that will be read and acted on by an application in a comment, as is frequently done in HTML documents, you should store it within the element structure of the actual XML document. Enhancing the readability of a complex DTD or temporarily disabling blocks of markup are effective uses of comments.

NOTE: The character sequence -- cannot be included within a comment block, except as part of the tag closing text. Because comments cannot be nested, commenting out a comment block is impossible. If large blocks of markup that include comments must be temporarily disabled, consider wrapping them in a CDATA section to cause the parser to read them as simple text instead of markup.

Processing Instructions

<?target [processing-instruction data]?>

Processing instructions provide an escape mechanism that allows an XML application to include instructions to an XML processor that are not part of the XML markup or character data. The processing instruction target can be any legal XML name, except xml in any combination of upper- and lowercase (see Chapter 2). Linking to a stylesheet to provide formatting instructions for a document is a common use of this mechanism. According to the principles of XML, formatting instructions should remain separate from the actual content of a document, but some mechanism must associate the two. Processing instructions are significant only to applications that recognize them.

The notation facility can indicate exactly what type of processing instruction is included, and each individual XML application must decide what to do with the additional data. No action is required by an XML parser when it recognizes that a particular processing instruction matches a declared notation. When this facility is used, applications that do not recognize the public or system identifiers of a given processing instruction target should realize that they could not properly interpret its data portion.

Character Encoding Autodetection

The XML declaration must be the very first item in a document so that the XML parser can determine which character encoding was used to store the document. A chicken-and-egg problem exists, involving the XML declaration's encoding="..." clause: the parser can't parse the clause if it doesn't know what character encoding the document uses. However, since the first five characters of your document must be the string <?xml (if it includes an XML declaration), the parser can read the first few bytes of your document and, in most cases, determine the character encoding before it has read the encoding declaration.

XML Declaration

<?xml version="1.0" [encoding="encoding-name"][ standalone="yes|no"]?>

The XML declaration serves several purposes. It tells the parser what version of the specification was used, how the document is encoded, and whether the document is completely self-contained or has references to external entities.

The XML declaration, if included, must be the first thing that appears in an XML document. Nothing, except possibly a Unicode byte-order mark, may appear before this structure's initial < character.

Version Information
... version="1.0" ...
The version information attribute denotes which version of the XML specification was used to create the current document. At this time, the only valid version is 1.0.

Encoding Declaration
... encoding="encoding-name" ...
The encoding declaration, if present, indicates which character-encoding scheme was used to store the document. Although all XML documents are ultimately handled as Unicode by the parser, the external storage scheme may be anything from an ASCII text file using the Latin-1 character set (ISO-8859-1) to a file with native Japanese characters.
XML parsers may also recognize other encodings, but the XML specification only requires that they recognize UTF-8 and UTF-16 encoded documents. Many parsers also support additional character encodings. For a thorough discussion of character-encoding schemes, see Chapter 26.

Standalone Declaration
... standalone="yes|no" ...
If a document is completely self contained (the DTD, if there is one, is contained completely within the original document), then the standalone="yes" declaration may be used. If this declaration is not given, the value no is assumed, and all external entities are read and parsed. It is possible to convert any document in which standalone="no" to a standalone document by replacing each external entity reference with the text contained in the external entity file.
From the standpoint of an XML application developer, this flag has no effect on how a document is parsed. However, if it is given, it must be accurate. Setting standalone="yes" when a document does require DTD declarations that are not present in the main document file is a violation of XML validity rules.

20.3.2. DTD (Document Type Definition)

Chapter 2 explained the difference between well-formed and valid documents. Well-formed documents that include and conform to a given DTD are considered valid. Documents that include a DTD and violate the rules of that DTD are invalid. The DTD is comprised of the DOCTYPE declaration and both the internal subset (declarations contained directly within the document) and the external subset (declarations that are included from outside the main document).

Parameter Entities

The parameter entity mechanism is a simple macro replacement facility that is only valid within the context of the DTD. Parameter entities are declared and then referenced from within markup or possibly from within other entity declarations. The source of the entity replacement text can be either a literal string or the contents of an external file. Parameter entities simplify maintenance of large, complex documents by allowing authors to build libraries of commonly used entity declarations.

Parameter Entity Declarations

<!ENTITY % name "Replacement text.">
<!ENTITY % name SYSTEM
     "system-literal">
<!ENTITY % name PUBLIC "pubid-literal" 
    "system-literal">

Parameter entities are declared within the document's DTD and must be declared before they are used. The declaration provides two key pieces of information:

The name of the entity, which is used when it is referenced
The replacement text, either directly or indirectly through a link to an external entity

Be aware that an XML parser performs some preprocessing on the replacement text before it is used in an entity reference. Most importantly, parameter entity references in the replacement text are recursively expanded before the final version of the replacement text is stored. Character references are also replaced immediately with the specified character. This replacement can lead to unexpected side effects, particularly when constructing parameter entities that declare other parameter entities. For full disclosure of how entity replacement is implemented by an XML parser and what kinds of unexpected side effects can occur, see Appendix D of the XML 1.0 specification. The specification is available on the World Wide Web Consortium web site (http://www.w3.org/TR/REC-xml#sec-entexpand ).

General Entities

General entities are declared within the document type definition and then referenced within the document's text and attribute content. When the document is parsed, the entity's replacement text is substituted for the entity reference. The parser then resumes parsing, starting with the text that was just replaced.

General entities are declared within the DTD using a superset of the syntax used to declare parameter entities. Besides the ability to declare internal parsed entities and external parsed entities, you can declare external unparsed entities and associate an XML notation name with them.

Internal entities are used when the replacement text can be efficiently stored inline as a literal string. The replacement text within an internal entity is included completely in the entity declaration itself, obviating the need for an external file to contain the replacement text. This situation closely resembles the string replacement macro facilities found in many popular programming languages and environments:

<!ENTITY name "Replacement text">

There are two types of external entities: parsed and unparsed. When a parsed entity is referenced, the contents of the external entity are included in the document, and the XML parser resumes parsing, starting with the newly included text. When an unparsed entity is referenced, the parser supplies the application with the unparsed entity's URI, but it does not insert that data into the document or parse it. What to do with that URI is up to the application. Any entity declared with an XML notation name associated with it is an external unparsed entity, and any references to it within the document must be made using attribute values of type ENITITY or ENTITIES:

<!ENTITY name SYSTEM 
    "system-literal">
<!ENTITY name PUBLIC 
    "pubid-literal" "system-literal">

Text Declarations

<?xml[ version="1.0"] encoding="encoding-name"?>

Files that contain external parsed entities must include a text declaration if the entity file uses a character encoding other than UTF-8 or UTF-16. This declaration would be followed by the replacement text of the external parsed entity.

NOTE: External parsed entities may contain only document content or a completely well-formed subset of the DTD. This restriction is significant because it indicates that external parameter entities cannot be used to play token-pasting games by splitting XML syntax constructs into multiple files, then expecting the parser to reassemble them.

Unparsed Entities

It may be necessary at times to include data in your XML document that should not be parsed. For instance, your XML document may need to include pointers to graphics files that will be used by an application. These files are logically part of the document, but should not be parsed. The XML language allows you to declare external unparsed entities that can be included as attribute values within the content of your document:

<!ENTITY name SYSTEM  
    "system-literal" NDATA notation_name >
<!ENTITY name PUBLIC "pubid-literal "
    "system-literal" NDATA notation_name >

To include unparsed entities, you must first declare a notation that will be referenced in the actual entity declaration:

<!NOTATION gif SYSTEM "images/gif">

Then declaring the entity itself is possible:

<!ENTITY bookcase_pic SYSTEM "bookcase.gif" NDATA gif>

As an unparsed general entity, it can be referenced only as an attribute value of type ENTITY or ENTITIES:

<picture src="bookcase_pic" type="gif"/>

When an XML parser parses this element, the information contained in the entity and notation declarations can be used to identify the actual type of data stored in the external entity. For example, a program could choose to display the contents of a GIF external entity on the screen, once the actual format is known.

NOTE: XLink and similar mechanisms are commonly used in place of unparsed entities.

External Subset

The document type declaration can include part or all of the document type definition from an external file. This external portion of the DTD is referred to as the external DTD subset and may contain markup declarations, conditional sections, and parameter entity references. It must include a text declaration if the character encoding is not UTF-8 or UTF-16:

<?xml[ version="1.0"] encoding="encoding-name"?>

This declaration (if present) would then be followed by a series of complete DTD markup statements, including ELEMENT, ATTLIST, ENTITY, and NOTATION declarations, as well as conditional sections, and processing instructions. For example:

<!ELEMENT furniture_item (desc, %extra_tags; user_tags?, parts_list, 
    assembly+)>

<!ATTLIST furniture_item
    xmlns CDATA #FIXED "http://namespaces.oreilly.com/furniture/"
>
...

Internal DTD Subset

The internal DTD subset is the portion of the document type definition included directly within the document type declaration between the [ and ] characters. The internal DTD subset can contain markup declarations and parameter entity references, but not conditional sections. A single document may have both internal and external DTD subsets, which, when taken together, form the complete document type definition. The following example shows the internal subset, which appears between the [ and ] characters:

<!DOCTYPE furniture_item SYSTEM "furniture.dtd"
[
<!ENTITY % bookcase_ex SYSTEM "Bookcase_ex.ent">

%bookcase_ex;

<!ENTITY bookcase_pic SYSTEM "bookcase.gif" NDATA gif>
<!ENTITY parts_list SYSTEM "parts_list.ent">
]>

Element Type Declaration

Element type declarations provide a template for the actual element instances that appear within an XML document. The declaration determines what type of content, if any, can be contained within elements with the given name. The following sections describe the various element content options available.

NOTE: Since namespaces are not explicitly included in the XML 1.0 recommendation, element and attribute declarations within a DTD must give the complete (qualified) name that will be used in the target document. This means that if namespace prefixes will be used in instance documents, the DTD must declare them just as they will appear, prefixes and all. While parameter entities may allow instance documents to use different prefixes, this still makes complete and seamless integration of namespaces into a DTD-based application very awkward.

Empty Element Type
<!ELEMENT name EMPTY>
Elements that are declared empty cannot contain content or nested elements. Within the document, empty elements may use one of the following two syntax forms:
<name [attribute="value" ...]/> <name [attribute="value" ...]></name>

Any Element Type
<!ELEMENT name ANY>
This content specifier acts as a wildcard, allowing elements of this type to contain character data or instances of any valid element types that are declared in the DTD.

Mixed Content Element Type
<!ELEMENT name (#PCDATA [ | name]+)*> <!ELEMENT name (#PCDATA)>
Element declarations that include the #PCDATA token can include text content mixed with other nested elements that are declared in the optional portion of the element declaration. If the #PCDATA token is used, it is not possible to limit the number of times or sequence in which other nested elements are mixed with the parsed character data. If only text content is desired, the asterisk is optional.

Constrained Child Nodes
<!ELEMENT name (child_node_regexp)[? | * | +]>
XML provides a simple regular-expression syntax that can be used to limit the order and number of child elements within a parent element. This language includes the following operators:

Operator

Meaning

Name

Matches an element of the given name

( ... )

Groups expressions for processing as sets of sequences (using the comma as a separator) or choices (using | as a separator)

?

Indicates that the preceding name or expression can occur zero or one times at this point in the document

*

Indicates that the preceding name or expression can occur zero or more times at this point in the document

+

Indicates that the preceding name or expression must occur one or more times at this point in the document

Attribute List Declaration
<!ATTLIST element_name [attribute_name attribute_type default_decl]*>
In a valid XML document it is necessary to declare the attribute names, types, and default values that are used with each element type.
The attribute name must obey the rules for XML identifiers, and no duplicate attribute names may exist within a single declaration.
Attributes are declared as having a specific type. Depending on the declared type, a validating XML parser will constrain the values that appear in instances of those attributes within a document. The following table lists the various attribute types and their meanings:

Attribute type

Meaning

CDATA

Simple character data.

ID

A unique ID value within the current XML document. No two ID attribute values within a document can have the same value, and no element can have two attributes of type ID.

IDREF, IDREFS

A single reference to an element ID (IDREF) or a list of IDs (IDREFS), separated by spaces. Every ID token must refer to a valid ID located somewhere within the document that appears as the ID type attribute's value.

ENTITY, ENTITIES

A single reference to a declared unparsed external entity (ENTITY) or a list of references (ENTITIES), separated by spaces.

NMTOKEN, NMTOKENS

A single name token value (NMTOKEN) or a list of name tokens (NMTOKENS), separated by spaces.

NOTATION Attribute Type
... NOTATION (notation [| notation]*) ...
The NOTATION attribute mechanism lets XML document authors indicate that the character content of some elements obey the rules of some formal language other than XML. The following short sample document shows how notations might be used to specify the type of programming language stored in the code_fragment element:
<?xml version="1.0"?> <!DOCTYPE code_fragment [ <!NOTATION java_code PUBLIC "Java source code"> <!NOTATION c_code PUBLIC "C source code"> <!NOTATION perl_code PUBLIC "Perl source code"> <!ELEMENT code_fragment (#PCDATA)> <!ATTLIST code_fragment code_lang NOTATION (java_code | c_code | perl_code) #REQUIRED> ]> <code_fragment code_lang="c_code"> main( ) { printf("Hello, world."); } </code_fragment>

Enumeration Attribute Type
... (name_token [| name_token]*) ...
This syntax limits the possible values of the given attribute to one of the name tokens from the provided list:
<!ELEMENT door EMPTY> <!ATTLIST door state (open | closed | missing) "open"> . . . <door state="closed"/>

Default Values
If an optional attribute is not present on a given element, a default value may be provided to be passed by the XML parser to the client application. The following table shows various forms of the attribute default value clause and their meanings:

Default value clause

Explanation

#REQUIRED

A value must be provided for this attribute.

#IMPLIED

A value may or may not be provided for this attribute.

[#FIXED ] "default value"

If this attribute has no explicit value, the XML parser substitutes the given default value. If the #FIXED token is provided, this attribute's value must match the given default value. In either case, the parent element always has an attribute with this name.

The #FIXED modifier indicates that the attribute may contain only the value given in the attribute declaration. Although redundant, it is possible to provide an explicit attribute value on an element when the attribute was declared as #FIXED. The only restriction is that the attribute value must exactly match the value given in the #FIXED declaration.

Special Attributes
Some attributes are significant to XML and must be declared and implemented in a particular way:

xml:space

The xml:space attribute tells an XML application whether the whitespace within the specified element is significant:

<!ATTLIST element_name xml:space (default|preserve) default_decl> <!ATTLIST element_name xml:space (default) #FIXED 'default' > <!ATTLIST element_name xml:space (preserve) #FIXED 'preserve' >

xml:lang

For an element's character content, the xml:lang attribute allows a document author to specify the human language for an element's character content. If used in a valid XML document, the document type definition must include an attribute type declaration with the xml:lang attribute name. See Chapter 5 for an explanation of language support in XML.

Notation Declaration
<!NOTATION notation_name SYSTEM "system-literal"> <!NOTATION notation_name PUBLIC "pubid-literal"> <!NOTATION notation_name PUBLIC "pubid-literal" "system-literal">
Notation declarations are used to provide information to an XML application about the format of the document's unparsed content. Notations are used by unparsed external entities, processing instructions, and some attribute values.
Notation information is not significant to the XML parser, but it is preserved for use by the client application. The public and system identifiers are made available to the client application so that it may correctly interpret non-XML data and processing instructions.

Conditional Sections
The conditional section markup provides support for conditionally including and excluding content at parse time within an XML document's external subset. Conditional sections are not allowed within a document's internal subset. The following example illustrates a likely application of conditional sections:
<!ENTITY % debug 'IGNORE' > <!ENTITY % release 'INCLUDE' > <!ELEMENT addend (#PCDATA)> <!ELEMENT result (#PCDATA)> <![%debug;[ <!ELEMENT sum (addend+, result)> ]]> <![%release;[ <!ELEMENT sum (result)> ]]>

Table 20-1. Predefined entities

20.3. XML Syntax

20.3.1. Global Syntax Structures

Character Encoding Autodetection

20.3.2. DTD (Document Type Definition)

20.3.3. Document Body

20.3.4. Namespaces

Operator	Meaning
Name	Matches an element of the given name
( ... )	Groups expressions for processing as sets of sequences (using the comma as a separator) or choices (using \| as a separator)
?	Indicates that the preceding name or expression can occur zero or one times at this point in the document
*	Indicates that the preceding name or expression can occur zero or more times at this point in the document
+	Indicates that the preceding name or expression must occur one or more times at this point in the document

Attribute type	Meaning
`CDATA`	Simple character data.
`ID`	A unique `ID` value within the current XML document. No two `ID` attribute values within a document can have the same value, and no element can have two attributes of type `ID`.
IDREF, IDREFS	A single reference to an element `ID` (`IDREF`) or a list of `IDs` (`IDREFS`), separated by spaces. Every `ID` token must refer to a valid `ID` located somewhere within the document that appears as the `ID` type attribute's value.
ENTITY, ENTITIES	A single reference to a declared unparsed external entity (`ENTITY`) or a list of references (`ENTITIES`), separated by spaces.
NMTOKEN, NMTOKENS	A single name token value (`NMTOKEN`) or a list of name tokens (`NMTOKENS`), separated by spaces.

Default value clause	Explanation
`#REQUIRED`	A value must be provided for this attribute.
`#IMPLIED`	A value may or may not be provided for this attribute.
`[#FIXED ]` `"`default value`"`	If this attribute has no explicit value, the XML parser substitutes the given default value. If the `#FIXED` token is provided, this attribute's value must match the given default value. In either case, the parent element always has an attribute with this name.